Fuses and antifuses are programmable electronic devices that are used in a variety of circuit applications. A fuse is normally closed, and when blown or programmed results in an “open” or increase in resistance. An antifuse is similar to a fuse in that it is programmable. However, an antifuse is normally open, having a capacitor-like structure. When an antifuse is blown or programmed, this results in a short, or decreased resistance.
There are many applications for fuses and antifuses. One particular application is for customizing integrated circuits (IC's) after production. One IC configuration may be used for multiple applications by programming the fuses and/or antifuses (e.g., by blowing or rupturing selected fuses and antifuses) to deactivate and select circuit paths. Thus, a single integrated circuit design may be economically manufactured and adapted for a variety of custom uses.
Fuses and antifuses may also be used to program chip identification (ID) after an integrated circuit is produced. A series of ones and zeros can be programmed in to identify the IC so that a user will know its programming and device characteristics, as examples.
Another exemplary application for fuses and antifuses is in memory devices, to improve yields using redundancy. By providing redundant memory cells on memory chips, the circuits or modules that are defective or not needed may be eliminated from circuit operation, thus improving the yield. This may be accomplished by programming fuses or antifuses to alter, disconnect or bypass active cells or circuits and allow redundant memory cells to be used in place of cells that are not functional. Similarly, information may be rerouted using fuses and/or antifuses.
Typically, fuses or fusible links are incorporated into an integrated circuit design, and then these fuses or fusible links are selectively programmed, e.g., blown or ruptured, by passing an electrical current of sufficient magnitude through the selected fuses to cause them to melt and break the connection. A common design for a fuse circuit is shown in FIG. 1. A prior art fuse 12 include two terminals 14 and 16, typically with one terminal 14 being larger than the other 16, as shown. The fuse 12 includes a necked section or link 15 disposed between the two terminals 14 and 16. The fuse 12 may comprise silicided polysilicon, other semiconductors, or metal, as examples. When a predetermined amount of current is applied between the terminals 14 and 16, the link 15 is severed or altered, such that the fuse 12 is an “open” or is changed (e.g., increased) in resistance.
A preferred mechanism for programming the fuse is to alter the fuse by electromigration rather than actual rupturing of the fuse. Electromigration occurs when a current of sufficient magnitude flows through the fuse material. Current flows in a first direction, and electrons flow in a second direction opposite from the first direction. The electrons push some of the fuse conductive material towards the second direction of the electron flow. This results in the changing of the resistance of the fuse link without completely breaking it apart; rather, electromigration results in an increase in the resistivity of the material comprising the fuse. Programming a fuse using electromigration is typically more controllable and reliable than severing a fuse link.
A fuse 12 that has been programmed or altered by electromigration is shown in FIG. 2. To program the fuse 12, a voltage is applied between terminals 14 and 16 to cause a current to flow from terminal 16 to terminal 14 (thus causing electrons to flow from terminal 14 to terminal 16). The fuse 12 exhibits increased resistance after being programmed, because portions of the link 15 and terminal 14 have been destroyed, e.g., some material is missing from the left side of the fuse link 15, and because a portion of the fuse link 15 material has been pushed from terminal 14 to terminal 16, in the direction of electron flow. While the nature of the fuse link 15 has been changed, the outline of the fuse 12 remains substantially intact after programming. The fuse 12 still has the same length and width after it is programmed, for example. Note that the terminals 14 and 16 include a plurality of contacts 24 and 26 disposed thereon.
Referring again to the prior art schematic shown in FIG. 1, a prior art two terminal fuse 12 is typically utilized in a circuit 10 by coupling one terminal 14 of the fuse 12 to a field effect transistor (FET) 18 having a gate, source and drain, and by coupling the other terminal 16 of the fuse 12 to a programming voltage Vp. The terminal 14 may be coupled to the FET 18 drain. The gate of the FET 18 is coupled to a select circuit 20, which is coupled to a clock signal CLK. The source of the FET 18 is coupled to a reference voltage or ground GND, as shown.
Typically, a large number of fuses 12 are used in a circuit (not shown), each having its own FET 18. The select circuit 20 is adapted to select and program the fuse 12 that is desired to be read or programmed. The information regarding the state of the fuse (open or closed) is stored in a latch 22 that is coupled to the terminal 14. The INPUT and NEXT signals are used to form a chain of fuses 12. In such a configuration the NEXT signal of one latch connects to the input of the next latch. The latch 22 may include sense circuitry to sense the state of the fuse 12, not shown.
The FET 18 is used to change the resistance of the fuse 12 by allowing causing current to flow through the fuse 12 when the FET 18 is selected. Information read from the fuse 12 is stored in the latch 22. For example, to program the fuse 12, when the correct clock CLK is received by the select circuit 20 and when a “1” is stored in the latch 22, indicating that the fuse 12 has been selected to be programmed or changed, then the FET 18 is turned on. When the FET 18 is turned on, due to the programming voltage VP applied to terminal 16 a current passes through the fuse 12 from terminal 16 to terminal 14. The current changes the resistance of the fuse 12 by changing the nature of the material that comprises the fuse 12. As described above, this change may comprise rupturing the link 15 or altering the link 15 and/or terminals 14 and 16 to change the resistance of the fuse 12. The initial resistance of the fuse 12 prior to programming may be around 100 ohms, and after programming, the resistance of the fuse 12 may range from 1000-10,000 ohms, as examples.
A disadvantage of the circuit 10 shown in FIG. 1 is that a large amount of current and/or voltage is required to alter the state of the prior art fuse 12. For example, 10 mA or more of current and a voltage of about 2.5 V between terminal 16 and ground may be required to program the fuse 12. Thus, there is a need in the art for a fuse design that can be programmed at lower voltage and current levels. Also, prior art fuse transistors must be relatively large to accommodate the current and hence the circuits occupy a large chip area. Thus, there is a need in the art for fuse devices requiring smaller programming currents that would result in smaller chip areas.